In manufacturing processes of a printed-circuit board, a flexible printed-circuit board and a multilayer printed-circuit board, release films are used in hot pressing a copper clad laminate or copper foil interposing a prepreg or a heat resistant film. And, in a manufacturing process of a flexible printed-circuit board, there is widely used a method of employing a release film in order to prevent the adhesion of a cover lay film to a pressing hot plate when hot press bonding the cover lay film or a reinforcing plate to a flexible printed-circuit board body provided with a electric circuit with a thermosetting adhesive or a thermosetting adhesive sheet through.
As a release film to be used in these uses, there have been employed such a fluorine base film, a silicon-coated polyethylene terephthalate film, a polymethylpentene film and a polypropylene film as disclosed in Japanese Kokai Publication Hei-2-175247 and Japanese Kokai Publication Hei-5-283862.
In recent years, from the growing social requests for environmental issues and safety, the ease of disposal treatment has been required in addition to the heat resistance withstanding hot press forming and a releasing property against a printed-circuit board or a hot pressing plate for these release films. Further, a non-contaminative property to a copper circuit has become important in order to improve the yields of products during hot press forming.
However, the fluorine base films having been heretofore used as a release film has the excellent heat resistance, releasing property and non-contaminative property, but had a problem that it is hard to burn and it generates a harmful gas in incineration for disposal after uses in addition to being expensive. And, the silicon-coated polyethylene terephthalate film and the polymethylpentene film might cause the contamination of a printed-circuit board, particularly a copper circuit, due to the migration of low molecular weight matter contained in silicon or constituents to impair quality. And, the polypropylene film had poor heat resistance and an insufficient releasing property.
And, in Japanese Kokai Publication 2003-313313, there is described a release film having at least one layer comprising a resin composition containing 100 parts by weight of resin comprising at least one species of a thermoplastic resin and/or a thermosetting resin and 0.1 to 100 parts by weight of laminar silicate. This release film does not produce a harmful gas as distinct from a fluorine base film and does not cause contamination due to the migration of the low molecular weight matter as distinct from a silicon-coated polyethylene terephthalate film and a polymethylpentene film and in addition it is extremely superior in heat resistance and a releasing property. However, this release film had a problem that this film was insufficient in point of the conformability to circuit patterns and there might be cases where it could not adequately conform to a printed-circuit board, a flexible printed-circuit board and a multilayer printed-circuit board and a flexible printed board, which are provided with complex electric circuits and have circuit patterns on their surfaces, in hot pressing. Further, since the laminar silicate necessarily has adherents of low molecular weight substances causing an outgas, there was a problem that the outgas resulting from this low molecular weight substance might be generated even slightly and might contaminate a substrate.